200 research outputs found
Transport and Phonon Damping in He
The dynamic structure function informs about the dispersion and
damping of excitations. We have recently (Phys. Rev. B {\bf 97}, 184520 (2018))
compared experimental results for from high-precision neutron
scattering experiment and theoretical results using the ``dynamic many-body
theory'' (DMBT), showing excellent agreement over the whole experimentally
accessible pressure regime. This paper focuses on the specific aspect of the
propagation of low-energy phonons. We report calculations of the phonon
mean-free path and phonon life time in liquid \he4 as a function of wave length
and pressure. Historically, the question was of interest for experiments of
quantum evaporation. More recently, there is interest in the potential use of
He as a detector for low-energy dark matter (K. Schulz and Kathryn M.
Zurek, Phys. Rev. Lett. {\bf 117}, 121302 (2016)). While the mean free path of
long wave length phonons is large, phonons of intermediate energy can have a
short mean free path of the order of m. Comparison of different levels of
theory indicate that reliable predictions of the phonon mean free path can be
made only by using the most advanced many--body method available, namely, DMBT
Superfluid 4He dynamics beyond quasiparticle excitations
The dynamics of superfluid 4He at and above the Landau quasiparticle regime
is investigated by high precision inelastic neutron scattering measurements of
the dynamic structure factor. A highly structured response is observed above
the familiar phonon-maxon-roton spectrum, characterized by sharp thresholds for
phonon-phonon, maxon-roton and roton-roton coupling processes. The experimental
dynamic structure factor is compared to the calculation of the same physical
quantity by a Dynamic Many-body theory including three-phonon processes
self-consistently. The theory is found to provide a quantitative description of
the dynamics of the correlated bosons for energies up to about three times that
of the Landau quasiparticles.Comment: 5 pages, 3 figure
Microscopic dynamics of superfluid 4He: A comprehensive study by inelastic neutron scattering
The dynamic structure factor of superfluid 4 He has been investigated at very low temperatures by inelastic neutron scattering. The measurements combine different incoming energies resulting in an unprecedentedly large dynamic range with excellent energy resolution, covering wave vectors Q up to 5 Å^(-1) and energies ω up to 15 meV. A detailed description of the dynamics of superfluid 4He is obtained from saturated vapor pressure up to solidification. The single-excitation spectrum is substantially modified at high pressures, as the maxon energy exceeds the roton-roton decay threshold. A highly structured multiexcitation spectrum is observed at lowenergies, where clear thresholds and branches have been identified. Strong phonon emission branches are observed when the phonon or roton group velocities exceed the sound velocity. The spectrum is found to display strong multiexcitations whenever the single excitations face disintegration following Pitaevskii's type a or b criteria. At intermediate energies, an interesting pattern in the dynamic structure factor is observed in the vicinity of the recoil energy. All these features, which evolve significantly with pressure, are in very good agreement with the dynamic many-body calculations, even at the highest densities, where the correlations are strongest.Fil: Beauvois, K.. Universite Grenoble Alpes. Institut Nanosciences et Cryogenie - Commissariat a L´Energie Atomique et Aux Energies Alternatives. Institut Nanosciences et Cryogenie; Francia. Institut Laue Langevin; FranciaFil: Dawidowski, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Fåk, B.. Institut Laue Langevin; FranciaFil: Godfrin, H.. Universite Grenoble Alpes. Institut Nanosciences et Cryogenie - Commissariat a L´Energie Atomique et Aux Energies Alternatives. Institut Nanosciences et Cryogenie; FranciaFil: Krotscheck, E.. State University of New York; Estados UnidosFil: Ollivier, J.. Institut Laue Langevin; FranciaFil: Sultan, A.. Universite Grenoble Alpes. Institut Nanosciences et Cryogenie - Commissariat a L´Energie Atomique et Aux Energies Alternatives. Institut Nanosciences et Cryogenie; Franci
The dispersion relation of Landau elementary excitations and the thermodynamic properties of superfluid He
The dispersion relation of the elementary excitations of
superfluid He has been measured at very low temperatures, from saturated
vapor pressure up to solidification, using a high flux time-of-flight neutron
scattering spectrometer equipped with a high spatial resolution detector
(10 'pixels'). A complete determination of is achieved, from
very low wave-vectors up to the end of Pitaeskii's plateau. The results compare
favorably in the whole the wave-vector range with the predictions of the
dynamic many-body theory (DMBT). At low wave-vectors, bridging the gap between
ultrasonic data and former neutron measurements, the evolution with the
pressure from anomalous to normal dispersion, as well as the peculiar
wave-vector dependence of the phase and group velocities, are accurately
characterized. The thermodynamic properties have been calculated analytically,
developing Landau's model, using the measured dispersion curve. A good
agreement is found below 0.85 K between direct heat capacity measurements and
the calculated specific heat, if thermodynamically consistent power series
expansions are used. The thermodynamic properties have also been calculated
numerically; in this case, the results are applicable with excellent accuracy
up to 1.3 K, a temperature above which the dispersion relation itself becomes
temperature dependent.Comment: 35 pages, 59 figures; Supplemental Material: Text, Tables, data file
Evidence for magnetic quasiparticle phase separation in a quasi-one-dimensional quantum magnet
Magnetic systems composed of weakly coupled spin-1/2 chains are fertile
ground for hosting the fractional magnetic excitations that are intrinsic to
interacting fermions in one-dimension (1D). However, the exotic physics arising
from the quantum many-body interactions beyond 1D are poorly understood in
materials of this class. Spinons and psinons are two mutually exclusive
low-energy magnetic quasiparticles; the excitation seen depends on the ground
state of the spin chain. Here, we present inelastic neutron scattering and
neutron diffraction evidence for their coexistence in SrCoVO
at milli-Kelvin temperatures in part of the N\'eel phase (2.4 T
H 3.9 T) and possibly also the field-induced spin density
wave phase up to the highest field probed (H 3.9 T,
H = 5.5 T). These results unveil a
novel spatial phase inhomogeneity for the weakly coupled spin chains in this
compound. This quantum dynamical phase separation is a new phenomenon in
quasi-1D quantum magnets, highlighting the non-trivial consequences of
inter-chain coupling.Comment: 8 pages, 6 figure
The electrophysiology of the betacell based on single transmembrane protein characteristics
The electrophysiology of betacells is at the origin of insulin secretion.
Betacells exhibit a complex behaviour upon stimulation with glucose including
repeated and uninterrupted bursting. Mathematical modelling is most suitable to
improve knowledge about the function of various transmembrane currents provided
the model is based on reliable data. This is the first attempt to build a
mathematical model for the betacell-electrophysiology in a bottom-up approach
which relies on single protein conductivity data. The results of previous
whole-cell-based models are reconsidered. The full simulation including all
prominent transmembrane proteins in betacells is used to provide a functional
interpretation of their role in betacell-bursting and an updated vantage point
of betacell-electrophysiology. As a result of a number of in silico knock-out-
and block-experiments the novel model makes some unexpected predictions:
Single-channel conductivity data imply that calcium-gated potassium currents
are rather small. Thus, their role in burst interruption has to be revisited.
An alternative role in high calcium level oscillations is proposed and an
alternative burst interruption model is presented. It also turns out that
sodium currents are more relevant than expected so far. Experiments are
proposed to verify these predictions.Comment: 28 pages, 5 figures, 54 references, 14 pages supplementary materia
Field-temperature phase diagram of the enigmatic Nd2(Zr1−xTix)2O7 pyrochlore magnets
By combining neutron scattering and magnetization measurements down to 80 mK, we determine the (H, T ) phase diagram of the Nd2(Zr1−xTix )2O7 pyrochlore magnet compounds. In those samples, Zr is partially substituted by Ti, hence tuning the exchange parameters and testing the robustness of the various phases. In all samples, the ground state remains all in/all out, while the field induces phase transitions toward new states characterized by two in–two out or one out–three in/one in–three out configurations. These transitions manifest as metamagnetic singularities in the magnetization versus field measurements. Strikingly, it is found that moderate substitution reinforces the stability of the all in/all out phase: the Néel temperature, the metamagnetic fields along with the ordered magnetic moment, are higher in substituted samples with x < 10%
Do All Lives Have the Same Value? Support for International Military Interventions as a Function of Political System and Public Opinion of the Target States
This research examined the support for international military interventions as a function of the political system and the public opinion of the target country. In two experiments, we informed participants about a possible military intervention by the international community towards a sovereign country whose government planned to use military force against a secessionist region. They were then asked whether they would support this intervention whilst being reminded that it would cause civilian deaths. The democratic or nondemocratic political system of the target country was experimentally manipulated, and the population support for its belligerent government policy was either assessed (Experiment 1) or manipulated (Experiment 2). Results showed greater support for the intervention when the target country was nondemocratic, as compared to the democratic and the control conditions, but only when its population supported the belligerent government policy. Support for the external intervention was low when the target country was democratic, irrespective of national public opinion. These findings provide support for the democracy-as-value hypothesis applied to international military interventions, and suggest that civilian deaths (collateral damage) are more acceptable when nondemocratic populations support their government's belligerent policy
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